This application claims priority to Korean Patent Application No. 2004-8926, filed on Feb. 11, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to image-sensing devices, and more particularly, to sub-sampling mode in solid-state image-sensing devices such as of a complementary metal-oxide semiconductor (CMOS) image sensor (CIS)-type.
2. Description of the Related Art
A CIS-type solid-sate image-sensing device is typically used within small portable electronic devices such as a camera of a mobile phone or a digital still camera. The CIS-type solid-state image sensing device converts images into electrical signals for further processing by a digital signal processor. The digital signal processor processes color image data (red, green, and blue signals) output from the solid-state image-sensing device for driving a display device such as a liquid crystal display (LCD).
During a sub-sampling mode of the CIS-type solid-state image-sensing device, a full-frame image signal is generated with lower resolution. The sub-sampling mode is carried out for example in a preview stage in which an image to be sensed is checked before being fully sensed or in an auto focus setting stage with faster signal processing of lower resolution.
FIG. 1 is a block diagram of a conventional CIS-type solid-state image-sensing device 100 that includes an active pixel sensor (APS) array 110, a row driver 120, and an analog-to-digital converter 130. The row driver 120 receives a control signal from a row decoder (not shown), and the analog-to-digital converter 130 receives a control signal from a column decoder (not shown). The solid-state image-sensing device 100 further includes a controller (not shown) that generates addressing signals for controlling timing and selection of pixels for outputting sensed image signals.
When the conventional CIS-type solid-state image-sensing device 100 is for generating colored image signals, a respective color filter is disposed on top of each pixel of the APS array 110. The most common color filter array has a Bayer color pattern in which filters of the colors red (R) and green (G) are alternately disposed on one row and in which filters of the colors G and blue (B) are alternately disposed on the next row. Such a Bayer color pattern is known to one of ordinary skill in the art of image sensing devices.
For high resolution, the APS array 110 includes over a million pixels for example. In the CIS-type solid-state image-sensing device 100, each pixel in the APS array 110 generates a respective image signal by sensing light of the respective color filter with a respective photodiode and converting such light into a respective electrical signal. Thus, the respective image signal output from each pixel of the APS array 110 is an analog signal of one of three colors, R, G, and B. The analog-to-digital converter 130 converts such an analog image signal into a digital signal.
In addition, the analog-to-digital converter 130 uses a correlated double sampling (CDS) method to further process the image signals from the APS array 110. Such a method is described, for example, in U.S. Pat. Nos. 5,982,318 and 6,067,113. For the CDS method, the analog-to-digital conversion is basically divided into two operations: receiving a reset signal and the image signals from the APS array 110, and then processing the reset signal and the image signals to generate digital signals representing the image sensed by the APS array 110.
For the CDS method for example, the analog-to-digital converter 130 subtracts the reset signal from the image signals before conversion into the digital signals. Such digital signals are output to a digital signal processor that may further interpolate the digital image signals. In addition, the digital signal processor generates driving signals appropriate for the resolution of a display device such as a LCD (liquid crystal display).
In a sub-sampling mode of the CIS-type solid-state image-sensing device 100, the image signals are output with lowered resolution. For example, for the APS array 110 having super extended graphics adapter (SXGA) resolution (1280×1024), regular image signals are output at the SXGA resolution. However, for the sub-sampling mode, image signals are output at a video graphics adapter (VGA) resolution (640×480). Also, if the APS array 110 is for ultra extended graphics adaptor (UXGA) resolution (1600×1200), image signals are output with a resolution even lower than the VGA resolution for reducing data processing in the sub-sampling mode.
For sub-sampling mode in the conventional CIS-type solid-state image-sensing device 100, image signals of only a specific row and column that are spaced at a predetermined distance are output to the analog-to-digital converter 130 for lowering the resolution. In the above-mentioned example, to lower the SXGA resolution to the VGA resolution, the image signal from one pixel is selected from among a 2×2 matrix of four pixels for the ½ resolution.
The image signals from the other non-selected pixels are ignored for reduced data processing. With such ignored image signals, a diagonal portion on the display image is not smoothly connected, resulting in aliasing noise that is displayed in a zigzag form.